Production of cellulose pulp



Patented June 2, 1953 ada, Montreal; Quebec, Canada-l Application.luiylfi, 1949, SerialNt'; 11152162 This invention relates to theproduction of cellulosepulp from plant materials The invention is hereindesoribed as applied to the: chemical pulping of Wo'o'cl chips but theprinciples and procedures involved: are appliireduction at the steampressure to; the lower cable,- in Whole in part, tothe pulping of limitto he: aeoomplished= inv as brief; period: of straw; bamboo andotherligno cellulosio matesial time sush for example, a e-51 or: 5:minutes. In by the sui'phate; sulphiteand other-knownohemie thispretreatment procedure the: optimum. re;- oai pulping processes; suitsare attained by raisingz the; steam; pressure it salient feature of theinvention: consists" in m to the upper limit as rapidiy as possible;con-.- che provision: ofapretre'atm'ent pi oeeduref where.-si'ste'ntwithuniform: heating oi the chips; to the by wood chips ofwidely varying: original: moisture temperature oi the surrounding steam:and; by content, preferably within the eommeroial range discontinuingthe introduotioniofi stearm and-ink 25 to 50%: original moisturecontent, are ion the rapi'zl pressure reducing: withdrawal brought-toasubstantiallyuniform moisture con:- or steam from the vessel; soon; theupper tent, corresponding to or closely approaching? the limit of steam:pressure is attained; When; the fibre saturation: point for wood; andare otherraising" lowering ofi the steam pressure, is wise conditioned-to' facilitate the: rapid" uni? carried out in this preferredi manner,theztotal form penetration thereof by an aqueous solut-iine requiredforthe pretreatment procedure tionof defibra-tlng reagentsoom'moniyreferred is-reduceol to aminimumiwhieh iszdesirable both to as thecooking liquor; In: an article published from the standpointof: economyQfi opexzationand in Industrial and Engineering Chemistry; vol.reduction ofithe l'engthof: time: thEwWVQQdLChiQS 1, No; 2', Aprill5';I929; L. Stains-m hasv pointed are exposed tothe acti'omofi thezsteam,theilatter out that the term fibre: saturation point! was considerationbeing important to ensure; oonrli; first used connection with wood todesignate tioning of the chips for rapidi, and& uniform the moisture.content below which turthen reimpregnation thereof by defibratingreagents c'iuction of moisture: caused changes ing thewith-theleast-possible: danger: of; impairing. the strength of 'thewoodInthissamearticle:Stamm paper making; properties. on. the chips. In:this also points out that the: absorption oi water by connection, t Sd pfi note that the; die;- woodbelongs to the same general class of sii'edconditioningof the chips by the: pretreatr phenomena as the absorptionofi various: liquids ment procedureorftheipresentlinuentionis not do:-

diferent" fibIOllS- material. and; very closely pendent on the: use? ashigh: maximum-1 steam resembles the absorption or water by otheircelipressures or on: prolonged exposure; of; the wood luiosic fibres.Water" contained in wood; fibre ehips-to-theaetiom of! the, steam. On:the con,-

in two forms; i. 6;, free Water and absorbed trary', the maximum: steam;pressure; employed water anti, accordingi to: Stamm,. the estimationinthe pretreatmentiproceduretmay bea; relatives point may be defined asthat. state: in which: the 'iylow superatmospheric pressure. anda thereis cavity" of the fibre is entirely free from: liquidnonecessityof-maintainingthe: maximiun steam moisture and its well issaturated? throughout; pressure for any appreciable period following Iti sat this saturation-r point that a1 wo'ocl fibre 49 attainmenttl'iereofi commences to shrink on drying and exhibits its practice ithasbeen found. that:- there' is; a maximumability for alesorpti'on and;vdiffusioni of wise-range: within which the values to which thederibrating reagents aqueous solution;steam-pressureandtemperature.aresraiseitcluslnthepretieatnrentproeedure-aifordedbythe ing the pretreatment proeedureabe varied present invention the wood chips ofi varying depending on thenature. an'di condition ofi the original moisture content are brought.to': a sub- Wood andcompliance: with spieoifie: requirements stantiallyuniform moisture content; closely apconcerning yield: and quality" of;the: pulp; to; be preachingthe: fibre saturation poiht. by packingproduced front the: pretreatedi chips; thechipsinto as. pressurevessel;introducing The important faotorsgoverning; thesei mi steam into thevesselto" expel air therefrom 50 of the upperand lower limits: or steam:pressure through suitable vent opening; closing the employed in thepretreatmentprooedulzeare time veritopening when the: vessei is oiearedof air minimum: impairment of the: pulp? making and; continuing theintroduction: of: steam until pronert'iesof the woods The: results onnumerous the vessel isfill'ed with steam at aipreti'etermmedexperimentsand sommerciai app'licationszofi the superatmospheriopressure; ant-1i their rapidly 1 3* 2 dosing-the steam pressureto apredetermined lower value by disoontinuing the supply: of" steam andventing the vessel through one on more steamoutlets" of sumcientcapacity tot enaole invention indicate that use? of? maximum steampressures substantially higher than 50 lbs. per square inch is notcommercially economical because of the length of time required to attainsuch higher pressures in commercial digesters. Moreover, the us of suchhigher pressures prolongs the time of exposure of the wood chips to thesteam and thereby increases the hazard of impairing the pulp makingproperties of the chips by hydrolysis, distillation and other adverseeifects of superatmospheric steam. Hence while usable pulp may beproduced by pretreatment of the wood chips at maximum steam pressuresexceeding 50 lbs. per sq. in., the operating time is not economical andthe yield and quality of the pulp produced from the pretreated chips isinferior to that obtained from chips pretreated at maximum temperaturesbelow or not exceeding 50 lbs. per sq. in. The optimum maximum pressurerecommended for the pretreatment procedure is approximately 20 lbs. persq. in. This pressure can be reached in a very few minutes in acommercial digester and if not maintained for an excessive period oftime before being reduced to the lower limit, gives entirelysatisfactory conditioning results with a wide margin of safety asregards avoidance of any appreciable impairment of the pulp makingproperties of the wood by the action of the steam. High maximumpretreatment pressures ranging from 20 lbs. to 50 lbs. per sq. in. havealso been used in procedures which gave an increased yield of uniformlycooked good quality pulp representing a definite improvement over theyield and quality of pulp obtained by conventional pulping proceduresnot employing the pretreatment prescribed by the present invention. Whenpretreating wood chips at maximum pretreatment pressures of the order of40 to 50 lbs. per sq. in. or higher, it is especially desirable that thesteam pressure he maintained at this upper limit only momentarily inorder to reduce to a minimum adverse effects of this high pressuresteaming on the structure of the cellulose in the Wood and on thesolubility of the lignin and other .binding substances in normal pulpingliquors.

The lower limit to which the pressure in the pressure vessel is reducedduring pretreatment of-the chips may be any pressure which canconveniently be arrived at within an allotted period of time suitablefor economical commercial operation and which will give the requireddifferential between the upper and lower limits of pressure. At thispoint, it may be noted that the efficiency of the pretreatment procedureincreases with an increase in the rate at which the pressure is droppedfrom the upper to the lower limit. The lower limit of pressure may besuperatmospheric, zero or subatmospheric.

The mechanism of the pretreatment procedure may be explained as follows:During filling of the pressure vessel with steam to establish thereinthe selected upper limit of steam pressure and temperature, the woodchips are heated to the temperature of the surrounding steam whichpenetrates the chips to some extent. During the ensuing rapid reductionof the steam pressure to the selected lower limit, the water within thewood chips is flashed into steam and flows out of the chips withconsiderable velocity. This high velocity outflow of steam from withinthe wood chips serves to efiiciently purge the latter of air, free waterand certain penetration-retarding solids which are entrained and removedby the outflowing steam.

In addition to efliciently purging the wood chips of air, free water andother penetration retarding substances, the pretreatment procedure hasthe peculiar characteristic that it results in the moisture content ofthe chips being brought to or very close to the fibre saturation pointby either increasing or decreasing the original moisture content of thechips. When the original moisture content of the chips is below thefibre saturation point the pretreatment adds the required amount ofmoisture in the form of absorbed water. When the original moisturecontent of the chips is above the fibre saturation point, thepretreatment brings the moisture content close to the fibre saturationpoint by removing free water.

Repetition of the pretreatment procedur is desirable and, in many cases,necessary to bring the moisture content of the pretreated chipssufiiciently close to the fibre saturation point and to efiectsubstantially complete purging of the chips of air, free water, andcertain other penetration-retarding substances. Under especiallyfavourable conditions a single application of the pretreatment proceduremay give commercially satisfactory results but, as a general rule, twoor more applications of the pretreatment procedure will be foundnecessary to obtain the optimum results. On the other hand excessiverepetition of the pretreatment procedure may have detrimental effects onthe cellulose and should be avoided.

From the foregoing discussion it will be seen that the predeterminedprocedure which constitutes the basic novel feature of the inventionserves, by its unique moisture-content regulating efiect and itseflicient removal of penetration-retarding substances, to place thechips in the optimum condition for rapidly absorbing and uniformlydistributing throughout their structure or tissue the defibratingreagents contained in the cooking or pulping liquor. It is alsoimportant to note that these results are obtained without in any wayadversely affecting the pulp making qualities of the chips.

After being pretreated in accordance with the present invention the woodchips are rapidly and uniformly impregnated with the cooking liquor andheated to a pulping reaction temperature by one of several alternativeprocedures, the selection of which is dependent on such factors as thenature or species of the pretreated chips, the kind of pulp desired, andthe type of cooking liquor used. These various impregnating and pulpingprocedures and the basis on which they are selected in accordance withthe nature and condition of the raw material and the kind of pulpdesired are hereinafter fully described with the aid of suitableexamples and with reference to the accompanying drawings, in which:

Figure l is a graph showing the moisture content regulating effect of atypical pretreatment procedure to which the raw material is subjected inaccordance with the invention prior to being impregnated with thecooking liquor and heated to a pulping reaction temperature; and

Fig. 2 is a sectional View of a typical digester or pressure vesselwhich may be employed in the commercial application of the invention.

The graph (Fig. 1) shows the moisture regulating efiect obtained in alaboratory experiment by subjecting Douglas fir chips of widely varyingoriginal moisture content to four successive applications of thepretreatment purging procedure.

each: application or the: pretreatment procedure the: cycle: of: raisingthe: stem pressure the pressurevessel'. to: the? upper. limit ('40 lbs.per square inch): andi quickly reducing: it: to therlowerf limit lbs;pensduarez inch; was' com:- pleted; in: approxi'matelv three minutess.It will be: HOtBdz that this-2 pretreatment incneasedl the moisture:contents oi. chips: having; an; content? of from; 0% to 25%; and.reduced to 401% the moisture: content of: chips; having. an;origi'nalmoisture: content of approximately 551%.c It will alscr be: seem thatin: the: very; extreme: case of chipshaving; am original: moisturecontent of 185%,, the-.- pretreatment. reduced the: moisture contentto;54-%. The originalimoisure-content of wood. chips: originallyemployed. in; the. produc of cellulose: pulp lies within: the: range of2 tor 55%- and-,.,in: this range;,the=pretnea;tment procedure iseffective..- to: bring; alL the chips to a uniform. moisture content of:approximately 40 WhiGhi is; very close: to the fibre saturation pointat. which; the chips exhibit their: maximum ability to: absorb: and.uniformls dis-tribute throughout theirstructure defibratin'g; reagentswhich are brought in contact.- witlr the: chips in aqueous solution.

Commercial. applications of; the invention have been carried outv in: adigester oi the type:- indicatedlat 5 inaFig. 2.. The? rawwoodzchips-a-re introduced'. into the digesten throughi the'top fillingopening.-v t-wi-thorwithout the: aid of conventional n" p in equipment.At same: time;, or at any other suitable time; steam-a is admitted: tothe: bottom of. the: digester by;- steam supply line 1;, steami valve 8'and tha digester: blowdownoutlet-- 9? which controlled; by; annormally.- closed blowdown valve In... When-.thadigester has beensuificientlyi packed chips; the filling opening 6 is closed by a coverll: andithaintroduetion of steam is continued until the air in thedigester has been expelled through a strainer 12, a vent passage I3 andan open vent valve l4 provided at thetop ofithedigesten. Whenall theair. hasbeen expelled. from the digester,. steam is dischargedthroughventivalve Ill whichislthen closed and steaming continued toattain, as quickly as. possible, the selected. upper limit. oi! steam.pressure and. temperature... The steam is then. immediately shut' offand. rapid. reduction ofthe. steam. pressure. tofithe lower limit isinitiated. withtthe. least-possible delay byexhausting steamv from. the.digester through a bottom strainer. l 5and. a. drain valve [6 whichcontrols a. drain conduit Ill communicating: with. a space It reservedbetweenv strainer. 15. and. the: lower conicalbottom portion- 1.9 of.the digester. When drain valve 16.- is' opened, vent. valve. M is. alsopreferably opened. to facilitate the. desired rapid reduction of thesteam pressure: within the disesten.

It: is important; that the? digester: have adequate provision. forvrapid. reductiorr. in steamv pressure: Manydigesters now: in: use. donothave adequate provision-.ror this purpose; and: if itis'. desiredtopracticethe-present:inventiorrin suclrdigesters it is; recommended. thatbe? altered, preferably along; the lines of. the: digester." described.herein. Thus in. some. instances it; may benecessarv=- to installv a.strainer such. as: the bottom strainer 15,. and; a. drain valve: andconduit. of. large ca?- pacity such. assthe. valve. Ht andzthc. conduitl1 shown: inthedrawings.

lncasesswhere itLisinecessa-ry to: repeat thesprertreatment; procedure:to tiring; the: wood. chips: to the fibrersaturatiompoint and; to:efficiently purge 6i them of air, free water am otherpenetration-retarding substances the expulsion of; air: and: the;heating: ofi the chips during: the initial. application. of; thepretreatment procedure enables. the: desired raising and lowering"; ofasteamipressure inisucceeding: applicatinnssof: such procedure: to beappreciably shortened; as: com;- pared'. with the.- time required by theinitial; prertreatment of. the chips.

The steam. which: discharged: through tent valve? Ht, during thechipiheating; andiair-exp'eiling stage; of; the: pre'treatmentiprocedureat a: tentperature of; approximatelwalz Theiintroducztion". of: steam:into." the digester followinggthe closure of vent valvei M discontinuedas soon as the. desired upper limiter; steamipressura and temperature:isneachedi. and this upper: limit should; notexceed; a steam; pressure;or. 405' lbsz. per square inch-z witlr a corresponding; temperature: ofC; and: should: not beheld atthis: upper limit. for any-P appreciablelength of. time: suificient to impair. the: paper-making qualities: ofthe pretmaat'ed': chips;

While: the upper. and; lower limitsiofi steam pressure employed: in the:pretreatment procedure be widely variedawithini therprescribedrange; theselection; of. theselimits': and. the extent, if any, to which thepretreatment procedureai'stresa posted, will obviously be: based: cmthe.- attainment of commercially satisfactory results in: the

shortest: possible. time; having; regard-to the nae ture-arreLconditionof.- the=ravw material; Various trials. have established the fact. that,in. a large number of cases, commercially? satisfactory results can beobtained, in the pretreatment of: the

chips, by varying the steam: pressure: from; an

upper. limit of. 2G lbseper, err. in. to a lower. limit of. 10 lbs; persqt in.

With. reference to repetitionof. the pretreats mentprocedure,it-has-also been. established; that wood with. moisture contentsnormally en:- countered as well. aswater soaked: woods. may beefiiciently: purged. of penetration-retarding; substances and. broughtto a. uniform; moisture: cone tent closely approaching, the fibresaturation point-by: from .twoto -four. applications of the pretreatmentprocedure.

When thepretreatment procedure is completed valves.- l 4, and. t6) areclosed and. cooking; liquor: is introduced intothe. digester throughaevalve. 2D to commence( the impregnation and pulping-re? action. stagesof the complete processfor: the productiozn of. cellulose pulp;

The impregnation andi pulping of. the wood chips may be carriedoutasasingle-stage operation in which impregnation-takes placewhile thacookingliquor in. the digester. isbeine gradually heated: up to attain.pulpingereaction temperatures: and pressures; Im this. case;, thepretreatment of the: chips ensures that, ..bw'the. time pulping:reaction conditions are obtained in the digester,, the; chips; will. beunitormly penetrated by the pulping; or chemicalareagentsiof the'coolsing liquor. to-a much greater degree thamheretoforelw-iththe resultthatzthe pulp produced will be: of. a. more uniform; character in thatitwill contain smaller: percentages: of overcooked: and undercookedfibres.- 'lihe: rapidity and. deg-rem of uniliormitw with: which the.pretreated. chips: are impregnated also. oifers the. possibility of.sately shortening: the cooking; cycle: by,- increasingethe rapidity"with which the: cooking liquor is: heated up to: aapulping; reaction.temperature; and' detcreasing; tha length. of time. during" which thechips are left in contact with the: c'ock-ingiliquon afterpulping-reaction conditions have been attained.

The impregnation and pulping of the chips may also be carried out insuch manner as to constitute essentially a two-stage operation.Precautions are taken to maintain the temperature of the cooking liquorbelow the pulping-reaction temperature until it has been .determinedthat the chips have absorbed and uniformly distributed throughout theirstructure the full amount of defibrating chemicals of the cooking liquorwhich the chips are capable of absorbing and distributing throughouttheir structure. The contents of the digester are then heated up toattain pulping reaction conditions of temperature and pressure. Thistwo-stage procedure gives a greater margin of safety with reference toensuring that all the chips are uniformly impregnated to the fullestpossible extent before being subjected to pulping-reaction conditions oftemperature and pressure. It also lends itself to withdrawal of asubstantial proportion of the cooking liquor following completeimpregnation of the chips and thus enables the heating of the contentsof the digester to attain pulping reaction conditions to be carried outwith greater rapidity, thus contributing to the saving of both heat andtime.

When the impregnation and pulping of the chips is carried out as atwo-stage process, as

above described, the partially exhausted liquor which is withdrawn fromthe digester when the chips are completely impregnated is fortified andbrought back to its original strength by the addition of white liquor.

When impregnation and pulping of the chips is carried out as a two-stageprocess, it has been found that pressure impregnation of the chipsserves, in conjunction with the pretreatment procedure, to provide theoptimum conditions for the most complete and uniform impregnation of thechips in the shortest possible time preceding heating of the impregnatedchips to pulping reaction temperatures. When pressure impregnation isresorted to, the cooking liquor is pumped into the digester throughvalve 20 and is placed under the required pressure either by means ofthe liquor pump or by injecting steam into the top of the digester inany suitable manner and preferably through the top strainer l2. pressureon the cooking liquor is maintained for a suitable period of time toensure the best pos sible penetration results. Part of the cookingliquor is then preferably withdrawn from the digester through the bottomstrainer 15 and valve I6 before heating up the contents of the digesterto attain pulping reaction temperatures and pressures. The volume ofliquor withdrawn from the digester following the impregnation of thechips maybe any desired proportion of the total volume of liquorintroduced into the digester. In this connection, it may be noted thatsince the chips have been fully impregnated, there is no necessity forretaining a large volume of excess liquor in the digester during theheating of the digester content to pulping reaction temperatures andconsiderable time and heat may therefore be saved by withdrawing fromthe digester, following impregnation of the chips, all but a very smallamount of the excess liquor. The small amount of excess liquor retainedin the digester is substantially uniformly distributed among the chipsduring the actual cooking operation by steam distribution and byconventional liquor ciroulating methods.

The

The pressure employed in the pressure impregnation of the chips may bevaried Within wide limits ranging, for example, from about 20 lbs. persquare inch to lbs. per square inch or higher. In a number of instancesthe chips were impregnated at a pressure of 100 lbs. per square inch andresults were entirely satisfactory. However, in other instances,commercially satisfactory results were also obtained by impregnating thechips at an impregnating pressure of 20 lbs. per square inch. So far ascan be determined from the results of numerous experimental andcommercial applications of the invention, there appears to be noparticular advantage in using impregnating pressures above 100 lbs. persquare inch but it is possible that extended applications of theinvention may disclose instances in which use of higher pressures maydevelop some advantages. The ratio of the strength of the partiallyexhausted liquor withdrawn from the digester upon completion ofimpregnation appears to vary according to the original strength of theliquor and the temperature at which the impregnation takes place andthis consideration should be borne in mind in determining the strengthof the original cooking liquor. Another advantage of the presentinvention is that it lends itself to the uniform pulping of a mixture ofwoods of different species which cannot be uniformly pulped byconventional pulping procedures. By carrying out the pretreatment,impregnation and pulping reaction procedures as prescribed by thisinvention, it has been found possible to produce a uniform pulp ofcommercially satisfactory quality from a mixture of jack pine and poplarwood chips mixed together in equal proportion by Weight.

The invention is further illustrated by the following examples:

EXAMPLE 1'.

Production of Icraft pulp from jack pine chips having an originalmoisture content within the normal commercial range The chips werepacked into the digester through opening 6 by a conventional steampacker (not shown), aided by introduction of steam into the bottom ofthe digester through steam valve 8. q The introduction of steam into thebottom. of the digester through valve 8 was effected at the maximum rateconsistent with prevention of the steam breaking through the chipcharge. When the digester was sufficiently packed with chips, thecharging opening 6 was closed by cover H with vent valve l4 open. Thesupply of steam through valve 8 was continued until a steam dischargetemperature of 212 F. was registered at vent valve Hi. The valve 64 wasthen closed and the introduction of steam through valve 8 continueduntil the steam pressure in the digester was raised to 20 lbs. per sq.in. This required approximately '7 minutes. The supply of steam throughvalve 8 was then immediately discontinued and valves 14 and I6 wereimmediately opened to reduce the steam pressure in the digester to 10lbs. per sq. in. as rapidly as possible. This reduction of steampressure required approximately 6 minutes. When the lower limit ofpressure was reached, the valves M and I6 were closed and steam valve 8again opened to again raise the steam pressure in the digester to 20lbs. per sq. in. This second raising of the steam pressure to thespecified upper limit reace-emu 9 uu-ired approximately minutes. Thevalve 2E5 was then immediately reclosed and the relief valves M and B"quickly open-ed to again reduce the steam pressure in the digester toI=1'-0 lbs. .per

sq. This secondreduotionloi thesteamapres -5 sure to the lower limitrequired. about minutes. In this case no further repetition of "thepretreatmehtproeed-ure was iound'nccessary toibring the chips to thefibre saturation :point sand 'Ebf) e'fiec't a satisfactory purging ofthe :c'hips of :air, free water and other "penetration-resistingzsubstances.

When the pretreatment prooedure was roompleted, the cooking liquormrrlpecl :into the ,digester "to "completely fill the same and theliquor pump was continued in operation tor a. iiurther :period of two tothree minutes. At the rendof this time, the level *record'er ion theliquor :tan'k showed :no fu'rther decrease volume ar-1d the liquorsupply pump was shut :oii'.

Following *the shutting 0ff of .ithe liquor supply pump, the cookingliquor in the digester wassub iiected'to mechanical pressure by *meansof steam introduced "into the top of the :digester.

pressure applyin steam actedas-aipistomon top the digester and returningit "to the iliquor tan l;

at the end of the pressure impregnation lot the chips. 'The'tota-lperiod -o'f timere'quired'for filling the digester with the -codkingjiiquor and eflecting the pressure impregnation of :the rchips wasapproximately 12 minutes and the temperature of impregnation was helowthe pulpingreaction'temperature. 7

When the cooking liquor "in the digester was -reduced to the selectedlevel, following completion oi the pressure impregnation of rthe ohips,l

steam was introduced into the :diges'ter to bring the contents thereofto a maximum temperature of approximatelyfifiil" F. l hisrequiredlappmxi- -mately minutes and the pulping readtion' temperaturethus established was maintained for a further period of approximately'two hours to complete "the cook.

"The cooking liquor used was a regular hra-ft cooking liquor ofapproximately 30% *stilphidity .and cont lug approximately .l'6f5'%active alkali.

This operation produced a good quality of .un'i- Jformly cooked pulpwith an increased .yield of ,irom 2% .to 3.2%.. The pulp .was easilywashed with less Water than is usuallylrequired .and conztaineda reducedamount of .lsnotter rejects. The steam consumption and thetimerequiredtocompiste the cook -was vappremlab-ly reduced rclue to thellow ratio -.of cooking diquor to wood :employed in the zdeiibratingreaction stage 'of theprocess. "slhegpretreatment 'of chips, plus the:low "liquor no mood ratio employed in the ideiibrating :reaction stageenabled the temperature of thesiig-ester "contents to he :raised ito thepulping reaction temperature *as rapidly as the system would rper- --mitthis "makes it possible to 'mcrease the digester turnover, that is tosay, in themun-rber of cooks 'coulp'let'edin 'eachiiigester over aperiod of such as '24 hours.

.i'I Production .0; Suzann ,puzzp with low liquor to wood ratio duringthe defibrating reaction stage of the process The chips were packed intodisaster {and were subjected to iour applications of the pretreatmentprocedure carried out :as described in Example I "The digesterwas thenfilled XVI-171178711 amount of sulphite cooking :liquor :swiieient to:completely submerge the chips and the fliquor was then subjected, 'forrs. "period "of ten :minutes, to aprcssureaof ioiirlbsxfper square'inchat afternperature below the deiibrating reaction temperature. Asufilcient amount oi the surplus cooking liquor was then immediatelywithdrawniromthe ldigester to reduce the liquor to wood ratio to about2.3 to 1, calculated on the basis of the bone dry weight of the wood.The contents of the digester were {thenheateiil to Ctin lfiminutes andthis temperature was maintained for hours. This operation produced atotal yield 53.2% of good quality uniformly cooked pulpwtdth a screenyield of approximately 53%. The pulp was QUi'bBBOft with no evidence ofburning.

. EXAMPLE III Production of ,sulphite pulp from western hemlock phi-p3In this case the-digester was packed with Wood .chipsiin thelsame manneras described mammp'le 'I ,'namel y,, with-the aid of a chippackerand theadmission of steam to theihottom of the .digester through the steamvalve "3. This required aperiod of,approximatelyjfiolininutes.

"When packing of the disaster with chips was completed, the chargingopening (6 WaSlChSCl cover H; vent valve M was opened; and theintroduction .o'f steam was continued .throu h valve I8 untilsteam wasdischarged thrcugh vent valve M at .a temperature 212 F. .This .requireda period voi approximately '8 minutes.

.As .soon as the .temperature of the steam .discharged through ventvalve 1 1 was .registeredat 21'2F., the valve M wasclosedandtheintroduction of steam through valve 8 continued until the steampressurein thedigester rcgisteredfi lbs. per sq. m. This required filproximately 1.0minutes. The-introduction ofsteam into the .digester was thendiscontinued and the steam pressure in the 'diges'ter reduced as rapidlyaspossible t0.3 ,per sq. in. by withdrawing steam therefrom. aghisreduction of pressure required approximately 5 minutes and thepretreatment procedure was not repeated.

Following the reduction of steam pressure .in the digesterto 3"l'hs'per'sq. in. at the endof the pretreatment procedure,'cookingacid'irom acuit- --able accumulator was pumped into the digesterand the acid supply pump was continued in .operation-un'tilthe level'inthe -accuxmilator had ceased to fall. Thepump pressurethus'establishedinthe 'digester after the latter was filled with'acidwas approximately 75' lbsk per "sq. in. and was "arrived --at in anacid charging period of approximately 30 minutes. By this time, thechips had taken up the maximum amoimt of acid which they were capable ofabsorbing and side relief of "the di- "gester was resorted to foraperiod'of -15 minutes. fl he temperature of the digester contents wasthenraised tOfiPDlOXllll-EISGIY Mo -C. the shortest possible time thatthe system would permit by -introduction :of steam. This required about21 05 ;minutes:and the temperature was'maintained cat-the :level of i'6. :for ':a further period "of '20 minutes. A further period of 30minutes was used up in reducing the pressure in the digester.

The digester used in Example III was a conventional sulphite digester ofapproximately ten tons accepted pulp capacity and was provided with aconventional hot acid cooking system. With a digester of this type thenormal time required for completion of a cock, as measured from thecommencement of steaming until the blowdown, was 7 hours.

The yield and quality of pulp produced by this operation wassubstantially the same as that obtained by the procedure described inExample II.

EXAMPLE IV Production of uniformly cooked kraft pulp from jack pine andpoplar wood chips mixed together in equal proportions In this case theprocedure followed, except as regards the use of a mixture of wood chipsof different species, was exactly as described in Example I.

In the pulping of mixed species of wood in accordance with the procedureprescribed in Ex-- ample IV, it appears that the uniform pulping resultsobtained are due to the fact that each chip is made to take up a miximumamount of liquid which is constant or approximately so for each species.

be made sufficient for the pulping reaction and therefore each chip is,in fact. its own digester. Thus the species more easily pulped does nothave access, after delignification, to a continuing supply of chemical.and so further degrading of the pulp does not take place. The result is,therefore, a uniform product from a mixture of species. It may be againemphasized that the conditioning of the chip to its constant moisturesaturation value, coupled with its ability to subsequently receive anadequate amount of chemical in solution is an essential feature of thisprocess.

This operation produced a good quality or uniformly cooked pulp of mixedspecies with the increased yield and other advantages pointed out inconnection with Example I. The poplar was not overcooked and the rejectswere normal.

Having thus described the principles of our invention and severalapplications thereof, it will be understood that various modificationsmay be resorted to within the scope and spirit of the invention asdefined by the appended claims.

We claim:

1. In the production of cellulose pulp from fibrous plant material, theprocess which consists of the steps or introducing plant material insubdivided condition into a pressure vessel; displacing air in saidpressure vessel with steam; subjecting said plant material within saidpressure vessel to a pro-treating operation which compries introducingsteam under pressure to establish within the capillary interstices ofthe plant material a mixture of air and steam until a pressure of fromabout five to about fifty pounds per square inch gage pressure isreached, then discontinuing the introduction of steam, and whileexcluding additional air from said pressure vessel forthwith rapidlywithdrawing said mixture of air and steam from said pressure vessel andfrom the interstices of said plant material until a pressure is reachedwithin the range of from atmospheric pressure to a gage pressure halfthat which existed when the introduction or" steam was discontinued;thereafter immersing said pro-treated plant material in cooking liquorwhilecontinuing The amount of chemical taken up, by I regulating theconcentration of the solution, can

12 to exclude air from said plant material; and then cooking saidpre-treated plant material.

2. The process of claim 1 in which the pretreating operation is carriedout a plurality of times before the plant material is immersed incooking liquor.

3. In the production of cellulose pulp from fibrous plant material, theprocess which consists of the steps of introducing plant material insubdivided condition into a pressure vessel; displacing air in saidpressure vessel with steam; subjecting said plant material within saidpressure vessel to a pre-treating operation which comprises introducingsteam under pressure to establish Within the capillary interstices ofthe plant material a mixture of air and steam until a pressure of fromabout five to about fifty pounds per square inch gage pressure isreached, then discontinuing the introduction of steam, and whileexcluding additional air from said pressure vessel forthwith rapidlywithdrawing said mixture of air and steam from said pressure vessel andfrom the interstices of said plant material until a pressure is reachedwithin the range of from atmospheric pressure to a gage pressure halfthat which existed when the introduction of steam was discontinued;thereafter immersing said pretreated plant material in cooking liquor ata temperature below that of defibration reaction While continuing toexclude air from said plant material; subjecting said plant material andcooking liquor to a substantial hydrostatic pressure to force saidcooking liquor into th interstices of said plant material; and thenheating said impregnated plant material to defibration reactiontemperature.

4. The process of claim 3 in which the pretreating operation is carriedout a plurality of times before the plant material is immersed incooking liquor.

5. In the production of cellulose pulp from fibrous plant material, theprocess which consists of the steps of introducing plant material insubdivided condition into a pressure vessel; displacing air in saidpressure vessel with steam; subjecting said plant material within saidpressure vessel to a pre-treating operation which comprises introducingsteam under pressure to establish within the capilliary interstices ofthe plant material a mixture of air and steam until a pressure of fromabout five to about fifty pounds per square inch gage pressure isreached, then discontinuing the introduction of steam, and whileexcluding additional air from said pressure vessel forthwith rapidlywithdrawing said mixture of air and steam from said pressure vessel andfrom the interstices of said plant material until a pressure is reachedwithin the range of from atmospheric pressure to a gage pressure halfthat which existed when the introduction of steam was discontinued;thereafter immersing said pro-treated plant material in cooking liquorwhile continuing to exclude air from said plant material, the quantityof cooking liquor being sufiicient to completely submerge saidpre-treated plant material; withdrawing a substantial portion of thecooking liquor surrounding said plant material; and then cooking saidpre-treated plant material.

6. The process of claim 5 in which the pretreating operation is carriedout a plurality of times before the plant material is immersed incooking liquor.

'7. In the production of cellulose pulp from fibrous plant material, theprocess which consists of the steps of introducing plant material insubdivided condition into a pressure vessel; displacing air in saidpressure vessel with steam; subjecting said plant material Within saidpressure vessel to a pre-treating operation which comprises introducingsteam under pressure to establish within the capillary interstices ofthe plant material a mixture of air and steam until a pressure of fromabout five to about fifty pounds per square inch gage pressure isreached, then discontinuin the introduction of steam, and whileexcluding additional air from said pressure vessel forthwith rapidlywithdrawing said mixture of air and steam from said pressure vessel andfrom the interstice of said plant material until a pressure is reachedwithin the range of from atmospheric pressure to a gage pressure halfthat which existed when the introduction of steam was discontinued;thereafter immersing said pre-treated plant material in cooking liquorwhile continuing to exclude air from said plant material, the quantityof cooking liquor being sufiicient to completely submerge saidpre-treated plant material; and then cooking said pretreated plantmaterial while completely submerged in cooking liquor.

8. The process of claim 2 in which the pretreating operation is carriedout a plurality of times before the plant material is immersed incooking liquor.

JAMES HAMILTON ROSS. JOHN SEMPLE HART. RICHARD KEVIN STRAPP,

OTHER REFERENCES Chemistry of Pulp and Paper Making, by Sutermeister, 3dEd, published by John Wiley & Sons, New York (1941) pp. 101 and 187.

Manufacture of Pulp and Paper, 3rd Ed., vol. III, section 5, page 34(1937), published by Me- Graw-Hill, New York.

1. IN THE PRODUCTION OF CELLULOSE PULP FROM FIBROUS PLANT MATERIAL, THEPROCESS WHICH CONSISTS OF THE STEPS OF INTRODUCING PLANT MATERIAL INSUBDIVIDED CONDITION INTO A PRESSURE VESSEL; DISPLACING AIR IN SAIDPRESSURE VESSEL WITH STEAM; SUBJECTING SAID PLANT MATERIAL WITHIN SAIDPRESSURE VESSEL TO A PRE-TREATING OPERATION WHICH COMPRISES INTRODUCINGSTEAM UNDER PRESSURE TO ESTABLISH WITHIN THE CAPILLARY INTERSTICES OFTHE PLANT MATERIAL A MIXTURE OF AIR AND STEAM UNTIL A PRESSURE OF FROMABOUT FIVE TO FIFTY POUNDS PER SQUARE INCH GAGE PRESSURE IS REACHED,THEN DISCONTINUING THE INTRODUCTION OF STEAM, AND WHILE EXCLUDINGADDITIONAL AIR FROM SAID PRESSURE VESSEL FORTHWITH RAPIDLY WITHDRAWINGSAID MIXTURE OF AIR AND STEAM FROM SAID PRESSURE VESSEL AND FROM THEINSTERSTICES OF SAID PLANT MATERIAL UNTIL A PRESSURE IS REACHED WITHINTHE RANGE OF FROM ATMOSPHERIC PRESSURE TO A GAGE PRESSURE HALF THATWHICH EXISTED WHEN THE INTRODUCTION OF STEAM WAS DISCONTINUED;THEREAFTER IMMERSING SAID PRE-TREATED PLANT MATERIAL IN COOKING LIQUORWHILE CONTINUING TO EXCLUDE AIR FROM SAID PLANT MATERIAL; AND THENCOOKING SAID PRE-TREATED PLANT MATERIAL.